Reproductive isolation of sympatric populations may result from divergent selection of populations in different environments, and lead to ecological specialisation. In Brittany (France), the gorse Ulex europaeus (Fabaceae, Genisteae), may be encountered in sympatry with one of the two other gorse species present: U. gallii and U. minor. A recent study based on morphological identification of seed predators of gorse has shown that two weevil species (Curculionoidea, Apionidae) infest gorse pods at different seasons and have different host ranges: Exapion ulicis infests U. europaeus in spring, whereas E. lemovicinum infests U. gallii and U. minor in autumn. Weevil populations may thus have diverged in sympatry. 2. As morphological identification of weevils is often difficult and some of the characters used may exhibit individual or environmental variation, mitochondrial and nuclear sequences of weevils collected within pods of the three gorse species in 10 populations of Brittany were used to reconstruct their phylogeny. 3. The results reveal that species differentiation based on morphological characters is confirmed by the two molecular data sets, showing that E. ulicis and E. lemovicinum are distinct species, and suggesting the absence of host races. Finally, E. ulicis was able to use U. gallii and U. minor pods in spring in some years in some populations, which appeared to depend on the availability of pods present during its reproductive period. 4. Divergence between E. ulicis and E. lemovicinum may have resulted from temporal isolation of reproductive periods of weevil populations followed by specialisation of insects to host phenology.

The cold hardiness of two closely related weevil species, Exapion ulicis and E. lemovicinum was studied in relation to their life cycles. These two seed-eating weevils reproduce on Ulex plant species with different fruiting phenologies. E. ulicis lays eggs in spring and overwinters as an adult while E. lemovicinum lays eggs in autumn and overwinters as a larva. Adult weevils were collected in natural populations of Brittany (Western France) and characterized with morphological and molecular tools before experiments. We showed that both weevil species exhibited low supercooling points (SCPs) with mean seasonal values below -17¬įC. Fresh mass, moisture content and sex were not correlated to supercooling ability. Weevils died upon freezing and the lower lethal temperatures (LLT) were within the range of SCP, indicating that both species are freezing intolerant. Comparison between species for SCP, LLT and survival to exposure at -8¬įC in winter showed a higher cold resistance for E. ulicis than for E. lemovicinum. In addition, the seasonal evolution of cold hardiness differed depending on the species. These features suggest that response to cold of weevils is linked to their life cycles, and thus to the life history of their host plants.

Exapion ulicis (Forster) and Exapion lemovicinum (Hoffmann) (Coleoptera: Curculionoidea: Apionidae) are seed predators of the three gorse species occurring in Brittany (France): Ulex europaeus L., Ulex gallii Planch., and Ulex minor Roth. (Fabaceae). Host-plant phenology plays a major role in the relationship between apionid weevils and their gorse species, because larvae develop within gorse pods and adults have to wait for pod dehiscence to be released. We monitored flowering and fruiting phenology of gorse species, weevil reproductive behaviour, and egg-laying patterns in six natural populations in the native area of these gorse species. At each site, U. europaeus, which flowers mainly in spring, was sympatric with one of two autumn flowering gorse species, U. gallii and U. minor. We noticed that E. ulicis laid eggs in spring and was restricted to U. europaeus whereas E. lemovicinum laid eggs in autumn and was restricted to the two autumn-flowering species U. gallii and U. minor. Therefore, host specificity depended on gorse phenology, and not on geographic proximity. In addition, the infested pod content showed that E. ulicis laid several eggs per pod and suggested that females chose pods with the highest numbers of seeds. In contrast, E. lemovicinum laid a single egg per pod and showed no preference for pods with many seeds. Finally, the impact of seed predation by E. ulicis was higher than that of E. lemovicinum.